Homogenization of light beam for spectral feature metrology

ABSTRACT

A metrology system is used for measuring a spectral feature of a pulsed light beam. The metrology system includes: a beam homogenizer in the path of the pulsed light beam, the beam homogenizer having an array of wavefront modification cells, with each cell having a surface area that matches a size of at least one of the spatial modes of the light beam; an optical frequency separation apparatus in the path of the pulsed light beam exiting the beam homogenizer, wherein the optical frequency separation apparatus is configured to interact with the pulsed light beam and to output a plurality of spatial components that correspond to the spectral components of the pulsed light beam; and at least one sensor that receives and senses the output spatial components.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.15/364,006, filed Nov. 29, 2016, now allowed, and titled HOMOGENIZATIONOF LIGHT BEAM FOR SPECTRAL FEATURE METROLOGY, which is incorporatedherein by reference in its entirety.

TECHNICAL FIELD

The disclosed subject matter relates to an apparatus for homogenizing alight beam in order to measure and analyze a spectral feature, such as,for example, bandwidth or wavelength, of the light beam.

BACKGROUND

In semiconductor lithography (or photolithography), the fabrication ofan integrated circuit (IC) includes performing a variety of physical andchemical processes on a semiconductor (for example, silicon) substrate(which is also referred to as a wafer). A photolithography exposureapparatus or scanner is a machine that applies a desired pattern onto atarget portion of the substrate. The wafer is irradiated by a light beamthat extends along an axial direction, and the wafer is fixed to a stageso that the wafer generally extends along a plane that is lateral (andorthogonal) to the axial direction. The light beam has a wavelength inthe deep ultraviolet (DUV) range, for example, from about 10 nanometers(nm) to about 400 nm.

The light beam is produced by an optical source. An accurate knowledgeof spectral features or properties (for example, bandwidth andwavelength) of the light beam can be used, for example, to enablecontrol of a minimum feature size or critical dimension (CD) at thewafer. The CD is related to the feature size that is printed on thewafer.

SUMMARY

In some general aspects, a metrology system is configured to measure aspectral feature of a pulsed light beam, The metrology system includes abeam homogenizer in the path of the pulsed light beam, an opticalfrequency separation apparatus in the path of the pulsed light beamexiting the beam homogenizer, and at least one sensor. The beamhomogenizer includes an array of wavefront modification cells, and eachwavefront modification cell includes a surface area that matches a sizeof at least one of the spatial modes of the light beam. The opticalfrequency separation apparatus is configured to interact with the pulsedlight beam and to output a plurality of spatial components thatcorrespond to the spectral components of the pulsed light beam. Thesensor receives and senses the output spatial components.

Implementations can include one or more of the following features. Forexample, the metrology system can also include a control systemconnected to an output of the at least one sensor and configured to:measure a property of the output spatial components from the opticalfrequency separation apparatus for one or more pulses; analyze themeasured property to calculate an estimate of the spectral feature ofthe pulsed light beam; and determine whether the estimated spectralfeature of the pulsed light beam is within an acceptable range of valuesof spectral features. The spectral feature can be a bandwidth of thepulsed light beam. The metrology system can also include a spectralfeature selection system optically connected to the pulsed light beam.The control system can be connected to the spectral feature selectionsystem; and if the control system determines that the estimated spectralfeature of the pulsed light beam is outside the acceptable range, thenthe control system can be configured to send an adjustment signal to thespectral feature selection system to modify the spectral feature of thepulsed light beam.

A surface area of a cell can match a mode size of the light beam if thecell surface area is between 0.5 and 1.5 times the area of the spatialmode. A surface area of a cell can match a mode size of the light beamif the cell surface area is between 0.9 and 1.1 times the area of thespatial mode.

The metrology system can also include an optical diffuser in the path ofthe light beam, wherein the beam homogenizer receives the light beamthat is output from the optical diffuser. The optical diffuser caninclude a microlens array.

The metrology system can include a beam separation device in the pathbetween a source that produces the light beam and a photolithographyexposure apparatus. The beam separation device can direct a firstpercentage of the light beam toward the beam homogenizer, and can directa second percentage of the light beam along the path toward thephotolithography exposure apparatus. The metrology system can alsoinclude an optical temporal pulse stretcher between the beam separationdevice and the beam homogenizer. The optical temporal pulse stretchercan be a passive optical element.

The beam homogenizer can include an array having a plurality ofwavefront modification cells. The beam homogenizer can include a lensthat receives the light beam output of the array. The beam homogenizercan include at least two arrays, each array having a plurality ofwavefront modification cells. The beam homogenizer can include a lensthat receives the light beam output of the at least two arrays. Themetrology system can include an actuator connected to one or more of theat least two arrays, and configured to adjust a distance between the atleast two arrays.

The homogenized beam plane can be at the focal plane of the lens. Themetrology system can include a spinning diffuser at the homogenized beamplane.

The lens can have a focal length that is large enough so that thespacing between diffraction spikes of the output light beam from thearray or the at least two arrays is greater than an area of the at leastone sensor that receives the output light beam from the beamhomogenizer. The wavefront modification cell array can be made ofcalcium fluoride, fused silica, aluminum fluoride, encapsulatedmagnesium fluoride, gadolinium fluoride, or sodium aluminum fluoride.

The wavefront modification cell array can include an array of lenses orlenslets.

The wavefront modification cell array can be a transmissive cell array.

The optical frequency separation apparatus can include one or moreetalons.

The light beam can have a plurality of wavelengths, at least some beingin the deep ultraviolet range. The size of the spatial mode of the lightbeam can correspond to a transverse area across the light beam in whichall points within the transverse area have a fixed phase relationship.

The beam homogenizer can be in the path of a pulsed light beam outputfrom a power amplifier of an optical source. The beam homogenizer can bein the path of a pulsed seed light beam output from a master oscillatorof an optical source.

In other general aspects, a metrology system is configured to measure aspectral feature of a pulsed light beam. The metrology system includes:a beam homogenizer in the path of the pulsed light beam, an opticalfrequency separation apparatus that receives the pulsed light beamexiting the beam homogenizer, and at least one sensor that receives andsenses the output spatial components. The beam homogenizer includes apair of arrays, each array having a plurality of wavefront modificationcells; and a lens. The cells of the pair of arrays are spaced and sizedso that each spatial mode of the pulsed light beam that passes throughthe beam homogenizer is projected to the same area at the focal plane ofthe lens. The optical frequency separation apparatus is configured tointeract with the pulsed light beam and to output a plurality of spatialcomponents that correspond to the spectral components of the pulsedlight beam.

Implementations can include one or more of the following features. Forexample, each cell can have a surface area that matches a size of aspatial mode of the light beam. The surface area of a wavefrontmodification cell can be between 0.5 and 1.5 times the area of thespatial mode. The size of the spatial mode of the light beam cancorrespond to a transverse area across the light beam in which allpoints within the transverse area have a fixed phase relationship.

The metrology system can include a control system connected to an outputof the at least one sensor, the control system can be configured to:measure a property of the output spatial components for one or morepulses of the light beam; analyze the measured property to calculate anestimate of the spectral feature of the pulsed light beam; and determinewhether the estimated spectral feature is within an acceptable range ofvalues of the spectral feature.

The metrology system can include an optical diffuser in the path of thelight beam, and the beam homogenizer can receive the light beam that isoutputted from the optical diffuser.

The metrology system can include a spinning diffuser at the focal planeof the lens.

The optical frequency separation apparatus can include one or moreetalons.

In other general aspects, a deep ultraviolet light source includes: anoptical source including at least one gain medium that produces a pulsedlight beam; a beam separation device that directs a first portion of thepulsed light beam along a metrology path and directs a second portion ofthe pulsed light beam along a lithography path, a metrology system inthe metrology path, and a beam delivery system in the lithography paththat receives the pulsed light beam from the optical source and directsthe pulsed light beam to a photolithography exposure apparatus. Themetrology system includes: a beam homogenizer in the path of the pulsedlight beam, the beam homogenizer having at least a pair of arrays, eacharray having a plurality of wavefront modification cells; a lens,wherein the cells of the pair of arrays are spaced and sized so thateach spatial mode of the pulsed light beam that passes through the beamhomogenizer is projected to the same area at the focal plane of thelens; an optical frequency separation apparatus that receives the pulsedlight beam exiting the beam homogenizer, and is configured to interactwith the pulsed light beam and to output a plurality of spatialcomponents that correspond to the spectral components of the pulsedlight beam; and at least one sensor that receives and senses the outputspatial components.

Implementations can include one or more of the following features. Forexample, the light source can include an optical temporal pulsestretcher between the beam separation device and the beam homogenizer.

The optical source can include: a first gain medium that is a part of amaster oscillator that produces a pulsed seed light beam; and a secondgain medium that is a part of a power amplifier that receives the pulsedseed light beam from the master oscillator and outputs the pulsed lightbeam. A beam homogenizer can be in a path of the pulsed seed light beamor a beam homogenizer can be in a path of the pulsed light beam outputfrom the power amplifier. A first beam homogenizer can be in the path ofthe pulsed seed light beam and a second beam homogenizer can be in thepath of the pulsed light beam output from the power amplifier.

In other general aspects, a method for measuring a spectral feature of alight beam includes homogenizing the light beam including projectingeach transverse spatial mode of the light beam to the same transversearea at a beam homogenization plane; interacting the homogenized lightbeam with an optical frequency separation apparatus that outputs spatialcomponents that correspond to the spectral components of the light beam;sensing the spatial components; measuring a property of the sensedspatial components; analyzing the measured properties to estimate aspectral feature of the pulsed light beam; and determining whether theestimated spectral feature of the pulsed light beam is within anacceptable range of spectral features.

Implementations can include one or more of the following features. Forexample, if it is determined that the estimated spectral feature of thepulsed light beam is outside the acceptable range, an adjustment signalcan be sent to a spectral feature selection system to modify thespectral feature of the pulsed light beam.

DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of a photolithography system producing apulsed light beam that is directed to a photolithography exposureapparatus;

FIG. 2 is a graph of an exemplary optical spectrum of the pulsed lightbeam produced by the photolithography system of FIG. 1;

FIG. 3 is a block diagram of an exemplary metrology system that measuresone or more spectral features of the pulsed light beam produced by thephotolithography system of FIG. 1;

FIG. 4A is a block diagram of an exemplary diagnostic apparatus of themetrology system of FIG. 3;

FIG. 4B is a block diagram of an exemplary coherence-area matchingapparatus of the diagnostic apparatus of FIG. 4A;

FIG. 5 is a block diagram of an exemplary diagnostic apparatus of themetrology system of FIG. 3, which uses a coherence-area matchingapparatus such as shown in FIG. 4B;

FIG. 6 is a schematic side cross sectional view and a transverse planview of an exemplary wavefront modification device that is used in thecoherence-area matching apparatus of any of FIG. 4A, 4B, or 5;

FIG. 7 is a schematic optical diagram showing an exemplary diagnosticapparatus of the metrology system of any of FIG. 3, 4A, 4B, or 5, aswell as a location of a beam homogenization plane;

FIG. 8 is a block diagram of an exemplary spectral detection system thatcan be used in the metrology system of FIG. 5;

FIG. 9 is a block diagram of an exemplary optical source that can beused in the photolithography system of FIG. 1;

FIG. 10 is a block diagram of an exemplary spectral feature selectionapparatus that can be used in the photolithography system of FIG. 1;

FIG. 11 is a block diagram of an exemplary control system that can beused in the photolithography system of FIG. 1;

FIG. 12 is a flow chart of an exemplary procedure performed by thephotolithography system of FIG. 1 to measure one or more spectralfeatures of the pulsed light beam;

FIG. 13 is a block diagram of an exemplary wavefront modification devicethat can be used in the coherence-area matching apparatuses of any ofthe Figures; and

FIG. 14 is a block diagram of another implementation of the opticalsource that includes a beam homogenizer that includes the coherence-areamatching apparatus and can be used in the photolithography system ofFIG. 1.

DESCRIPTION

Referring to FIG. 1, a photolithography system 100 includes an opticalsource 105 that produces a pulsed light beam 110 that is directed to alithography exposure apparatus 115 for patterning microelectronicfeatures on a wafer 120. The photolithography system 100 uses a lightbeam 110 having a wavelength in the deep ultraviolet (DUV) range, forexample, with wavelengths of between about 10 nanometers (nm) and about400 nm. The wavelength can be, for example, 248 nm or 193 nm. The sizeof the microelectronic features patterned on the wafer 120 depends onthe wavelength of the light beam 110, with a lower wavelength resultingin a smaller minimum size of the microelectronic feature. When thewavelength of the light beam 110 is 248 nm or 193 nm, the minimum sizeof the microelectronic features can be, for example, 50 nm or less. Thefocus location of the pulsed light beam 110 at the wafer 120 correlateswith the wavelength of the light beam 110. Moreover, the bandwidth ofthe light beam 110 can impact the critical dimension (CD) of thesefeatures.

The bandwidth that is measured or determined and used for analysis andcontrol of the pulsed light beam 110 can be the actual, instantaneousbandwidth of its optical spectrum 200, as shown in FIG. 2. The opticalspectrum 200 contains information about how the optical energy or powerof the light beam 110 is distributed over different wavelengths (orfrequencies). The optical spectrum 200 of the light beam 110 is depictedin the form of a diagram where the spectral intensity (not necessarilywith an absolute calibration) is plotted as a function of the wavelengthor optical frequency. The optical spectrum 200 can be referred to as thespectral shape or spectrum of the light beam 110. Spectral properties orfeatures of the light beam 110 include any aspect or representation ofthe optical spectrum. For example, bandwidth is a spectral feature. Thebandwidth of the light beam is a measure of the width of the spectralshape, and this width can be given in terms of wavelength or frequencyof the laser light. Any suitable mathematical construction (that is,metric) related to the details of the optical spectrum 200 can be usedto estimate a value that characterizes the bandwidth of the light beam.For example, the full width of the spectrum at a fraction (X) of themaximum peak intensity of the spectral shape (referred to as FWXM) canbe used to characterize the light beam bandwidth. As one example, in acommonly used spectral-shape characterization, the fraction X is 50% andthe respective metric is commonly referred to as the full width at halfmaximum (FWHM). As another example, the width of the spectrum thatcontains a fraction (Y) of the integrated spectral intensity (referredto as EY) can be used to characterize the light beam bandwidth. In oneexample in common use for characterizing the spectral properties of thelight beam 110, the fraction Y is 95%.

Various disturbances (such as, for example, density or pressure of thegain medium or media in the optical source 105, temperature gradients ofoptical components, pressure gradients, optical distortions) act on theoptical source 105 and the light beam 110 to modify the spectralproperties or features of the light beam 110. For example, chromaticaberration caused by optical components that interact with the lightbeam 110 can cause an increase in the bandwidth of the light beam 110.Thus, the photolithography system 100 includes other components, suchas, for example, a spectral feature selection system 130, at least onemeasurement (or metrology) system 170, and a control system 185, thatare used to determine the impact of the disturbances on the light beam110 and to correct for the effect of such disturbances on the light beam110.

Because of the disturbances, the actual spectral feature (such as thebandwidth or the wavelength) of the light beam 110 at the wafer 120 maynot correspond to or match with the desired spectral feature. Thus, theactual spectral feature (such as a bandwidth) of light beam 110 ismeasured or estimated during operation by estimating a value of a metricfrom the optical spectrum 200 so that an operator or an automated system(for example, a feedback controller) can use the measured or estimatedbandwidth of the light beam 110 to adjust the properties of the opticalsource 105 and to adjust the optical spectrum of the light beam 110.

Referring to FIG. 3, to this end, the metrology system 170 includes abeam separator 160 and a diagnostic apparatus 165. The diagnosticapparatus 165 receives a light beam 110′ that is separated from thelight beam 110 by the beam separator 160. The beam separator 160 isplaced in a path between the optical source 105 and the photolithographyexposure apparatus 115. The beam separator 160 directs the light beam110′ (which is a first portion or percentage of the light beam 110) intothe diagnostic apparatus 165 and directs a second portion or percentageof the light beam 110 toward the exposure apparatus 115. In someimplementations, the majority of the light beam 100 is directed in thesecond portion toward the exposure apparatus 115. For example, the beamseparator 160 directs a fraction (for example, 1-2%) of the light beam110 into the diagnostic apparatus 165 and thus the light beam 110′ hasabout 1-2% of the power of the light beam 110. The beam separator 160can be, for example, a beam splitter.

The diagnostic apparatus 165 includes a spectral detection system 310that measures the spectral feature or features (such as the bandwidthand/or the wavelength) of the light beam 110 based on information aboutthe optical spectrum 200 of the light beam 110′. As discussed herein,the spectral detection system 310 include a spectrometer (such as anetalon spectrometer) that interacts with the light beam 110′ and outputsspatial components that correspond to the spectral components of thelight beam 110′, and a sensor that estimates the spectral feature orfeatures based on the outputted spatial components.

In order to uniformly sample the spectral content of the light beam 110′at the sensor, to evenly distribute the intensity of the light beam 110′at the sensor, and to provide a more accurate measurement of thespectral feature from the sensor, the diagnostic apparatus 165 includesa beam homogenizer 305 that is a part of a beam preparation system 300.The beam homogenizer 305 includes a coherence-area matching apparatus315 that is configured to reduce speckle noise and to improve beamhomogenization of the light beam 110′ impinging upon the sensor of thespectral detection system 310. The coherence-area matching apparatus 315mixes different spatial components of the light beam 110′ and smoothesout the intensity profile of the light beam 110′ prior to the light beam110′ entering the etalon spectrometer. Moreover, the coherence-areamatching apparatus 315 modifies the light beam 110′ so that its spatialmodes (which are its transverse electromagnetic modes) are overlappingat a beam homogenization plane (BHP) prior to entering the spectraldetection system 310. The coherence-area matching apparatus 315 reducesthe spatial coherence of the light beam 110′ before the light beam 110′enters the spectral detection system 310.

As shown in FIGS. 4A and 4B, the coherence-area matching apparatus 315includes at least one array 416 of wavefront-modification cells 418. Thearray 416 is arranged to be perpendicular to the direction of the beampath; in this example, the beam path is designated as the Z direction.Each cell 418 is an optical element that modifies the wavefront of thelight beam 110′. For example, each cell 418 can be a refractive opticalelement such as a lens having a convex surface, and thus, the array 416can be a microlens array.

In some implementations, the microlenses 418 of the array 416 arearranged in a periodic two-dimensional grid to form the array, with thedistances between the adjacent centers of microlenses 418 (taken alongthe X-Y plane perpendicular to the Z direction) being separated by astandard distance, which is referred to as a pitch, P.

Moreover, each cell 418 has an area (A(C)) taken along the X-Y plane.The area A(C) of a cell 418 is mathematically related to the pitch P ofthe array 416, the pitch P of the array 416 being the shortest distancebetween the centers of adjacent cells in the X-Y plane. The area A(C) ofeach cell 418 is matched to a size (for example, an area A(SM)) of oneor more of the transverse spatial modes 417 of the light beam 110′. Thesize or area A(SM) of the transverse spatial mode 417 is an area takenalong the plane (the X-Y plane) that is perpendicular to the beam path(the Z direction).

In some implementations, it is possible to use a random or pseudo-randomdistribution of areas A(C) for the cells 418 of the array 416. Forexample, the area A(C) of each cell 418 could be matched to thecoherence properties of the light beam 110′ and thus the area of aparticular cell 418 in the array 416 could be distinct from the area ofother cells 418 of the array 416.

The transverse spatial modes are electromagnetic field distributionsthat reproduce themselves after one round trip within the resonator orresonators of the optical source 105. Because of the geometry andconfiguration of the resonators within the optical source 105, thetransverse spatial modes can have complicated intensity distributionsand may not be well defined. Each transverse spatial mode has a distinctwavelength and the light beam 110′ can have on the order of 1000-2000transverse spatial modes, depending on the geometry and configuration ofthe optical source 105. For example, an estimated size or area of atransverse spatial mode of the light beam 110′ is 0.7 mm×0.1 mm in theorthogonal transverse directions at the output of the beam separator160. If the overall transverse size of the light beam 110′ is 12.5mm×12.5 mm, then there would be about 1800 coherence cells in the lightbeam 110′ at the output of the beam separator 160. The exemplarytransverse spatial mode 417 of the light beam 110′ that is shown in FIG.4B is a view taken along the Z direction, and is purely a schematicrepresentation of a rather simple transverse mode that is shown forillustration purposes only and may not be an actual intensitydistribution produced in any of the modes of the light beam 110′.Additionally, the array 416 and cells 418 are not drawn to scale.

The area A(C) of the cell can be considered to “match” the transversespatial mode size A(SM) if the cell area A(C) is within 0.5 to 1.5 times(for example, within 0.9 to 1.1 times) the spatial mode size A(SM). Bymatching the cell area A(C) to the spatial mode size A(SM), it becomespossible to project all of the spatial modes of the light beam 110′ tothe same area at a beam homogenization plane downstream of thecoherence-area matching apparatus 315. A closer match (for example,within 0.9 to 1.1 times) between the cell area A(C) and the transversespatial mode size A(SM) can be beneficial in some situations, forexample, situations in which the transverse spatial modes are moreclearly defined and/or not overlapping with each other.

The spatial mode size A(SM) can be determined by estimating a spatialcoherence area or size of the light beam 110′ because within a singlespatial mode 417, there is coherence, and thus, all points within thearea of the spatial mode 417 have a fixed phase relationship with eachother. The spatial coherence area can be determined by measuringinterference fringes between two pinholes separated by varying distanceand placed in the path of the light beam 110′.

As also shown in FIG. 4A, the beam homogenizer 305 can include otherelements or components for modifying aspects of the light beam 110′. Forexample, the beam homogenizer can also include a pulse stretcher system420, a diffuser system 425, and a spatial adjustment system 430.

The pulse stretcher system 420 includes a pulse stretcher that opticallyacts on the light beam 110′ to increase the duration of each of thepulses in the light beam 110″ without introducing significant losses sothat the peak power of the light beam 110′ is reduced without reducingits average power. The pulse stretcher system 420 acts on the light beam110′ prior to the light beam 110′ entering the coherence-area matchingapparatus 315 to further reduce the optical speckle noise that can befound at the homogenized beam plane. The pulse stretcher system 420 isan optical and passive configuration of optical elements that split theamplitude of the pulse of the light beam 110′ into split portions,introduce optical delays among these split portions, and then recombinethese temporally-delayed portions of the pulse to provide a temporallystretched pulse of the light beam 110′ at the output. In this way,different temporal portions of the pulse that are not coherent arecombined, and the speckle noise of the light beam 110′ is furtherreduced and therefore the spatial uniformity of the light beam 110′ isimproved. As discussed in greater detail below, the pulse stretchersystem 420 can therefore include optical components such as beamsplitters and reflective optics. The reflective optics can be flatmirrors or curved (for example, concave or convex) mirrors that could beconfocal. The delay introduced in the split portion of the pulseproduced by the pulse stretcher system 420 is equal to or longer thanthe fast temporal component of the light beam 110′. For example, a pulseduration of the light beam 110′ from the optical source can be about 40ns. Moreover, in some implementations, test data indicates that, at anygiven moment, the pulse is temporally coherent with other moments in thepulse that fall within 2.5 ns of that given moment, but the pulse hassignificantly reduced coherence with moments in the pulse that aredelayed by more than 2.5 ns. Thus, the coherence time (which is thedelay over which the phase or amplitude of the pulse wanders by asignificant amount) is about 2.5 ns in this example. In this example,the delay introduced in the split portion can be about 2.5 ns and thetotal path length that the split portion takes through the pulsestretcher system 420 on one pass can be on the order of tens ofcentimeters (cm) or about 70-80 cm. An example of a pulse stretchersystem 420 is discussed below with reference to FIG. 5.

The diffuser system 425 includes one or more optical elements that areconfigured to evenly diffuse the light beam 110′ prior to the light beam110′ entering the coherence-area matching apparatus 315. The diffusersystem 425 causes the light beam 110′ to spread evenly across thecoherence-area matching apparatus 315, thus minimizing or removing highintensity bright spots. The diffuser system 425 alters the angulardivergence of the light beam 110′ in a manner that ensures that theangular divergence of the light beam 110′ output from the diffusersystem 425 is less than an acceptance angle of the array 416 within thecoherence-area matching apparatus 315. For example, the diffuser system425 can modify the angular divergence of the light beam 110′ so that theangular divergence is much smaller than (for example, 20-40% less than)the acceptance angle of the array 416. The diffuser system 425 smoothesout or otherwise mitigates diffraction spikes that can sometimes beproduced by the coherence-area matching apparatus 315. The diffusersystem 425 creates multiple laterally (that is, spatially along adirection perpendicular to the direction of the light beam 110′) shiftedcopies of the diffraction spikes, which then smoothes out the intensityprofile of the light beam 110′ at the image plane (in the spectraldetection system 310). The diffuser system 425 can be a microlens arrayor a diffractive optic (which can be transmissive or reflective). Thediffuser system 425 can be a stationary or fixed microlens array or adiffractive optic. An example of a diffuser system 425 is discussedbelow with reference to FIG. 5.

The spatial adjustment system 430 is placed at the output of thecoherence-area matching apparatus 315 and it works to refract the lightbeam 110′ to spread out the spacing between diffraction spikes causeddue to the periodic nature of the coherence-area matching apparatus 315.In this way, the spacing between the diffraction spikes can be increasedby the spatial adjustment system 430 so that the spacing is larger thana region of interest of the sensor within the spectral detection system310. The spatial adjustment system 430 can be a lens that is positionedso that its focal plane overlaps the beam homogenization plane of thecoherence-area matching apparatus 315. An example of a spatialadjustment system 430 is discussed below with reference to FIG. 5.

Referring to FIG. 5, an exemplary metrology system 570 is shown. In themetrology system 570 of FIG. 5, the diagnostic apparatus 565 receivesthe light beam 110′ that has been separated from the primary light beam110 by the beam separator 560. The diagnostic apparatus 565 includes abeam preparation system 500 having a beam homogenizer 505 that includesa pulse stretcher system 520, a diffuser system 525, a coherence-areamatching apparatus 515, and a spatial adjustment system 530. All of theoptical components within the metrology system 570 are made of materialsand coatings that are configured to operate in a range of wavelengthsthat corresponds to the wavelength of the light beam 110′, for example,in the DUV wavelength range.

The pulse stretcher system 520 includes a beam splitter 521 that splitsthe amplitude of the pulse of the light beam 110′ into amplitudeportions, and circulates the split portions around a ring using a set ofmirrors 522A, 522B, 522C, 522D. After circulating around the ring, thetemporally-delayed portion of the split portion exits the pulsestretcher system 520 and is recombined with split portions that aretransmitted through the beam splitter 521. The mirrors 522A, 522B, 522C,522D can be flat or curved.

The diffuser system 525 is a microlens array that is placed at theoutput of the pulse stretcher system 520 and before the coherence-areamatching apparatus 515. As previously discussed, the diffuser system 525diffuses the light beam 110′ and also alters the angular divergence ofthe light beam 110′ in a manner that ensures that the angular divergenceof the light beam 110′ is less than an acceptance angle of the arraywithin the coherence-area matching apparatus 515.

In some implementations, the coherence-area matching apparatus 515includes a pair of wavefront modification devices 516A, 516B. Eachdevice 516A and 516B includes a two-dimensional array of wavefrontmodification cells (such as shown in FIG. 4). For example, as shown inFIG. 6, the coherence-area matching apparatus 615 includes, as thewavefront modification devices 516A and 516B, microlens arrays 616A and616B. Each array 616A, 616B includes, as the wavefront modificationcells, respective sets of microlenses 618A, 618B, each microlens 618A,618B refracting the light of the light beam 110′. In FIG. 6, only threemicrolenses 618A, 618B are referred to and there can be more or fewermicrolenses in the array. The microlenses 618A of the array 616A arearranged in a periodic two dimensional grid to form the array, with thedistances between the adjacent centers of microlenses 618A beingseparated by a standard distance, which is referred to as a pitch, P.

As discussed previously, the arrays 616A and 616B are arranged to beperpendicular to the direction of the beam path; in this example, thebeam path is designated as the Z direction and thus the array extendsalong the X-Y plane. Moreover, the microlenses 618A of the array 616Aare aligned with the microlenses 618B of the array 616B along the X-Yplane. Each microlens is a small lens, having a diameter or length thatis less than a millimeter (mm) and often as small as 10 micrometers(μm). A single microlens is a single element with one plane surface andone spherical convex surface to refract the light. The microlenses ofthe array 616A and 616B are applied to a support such as respectivesubstrate 619A and 619B. The microlenses 618A, 618B and the substrates619A, 619B are made of a material that is transmissive to the wavelengthrange of the light beam 110′. In some implementations, the microlenses618A, 618B and the respective substrates 619A, 619B are made of calciumfluoride.

In other implementations, the coherence-area matching apparatus 315includes a single wavefront modification device (such as the device516A), and the single wavefront modification device 316 functions withother aspects of the diagnostic apparatus 165 to homogenize the beam110′ in order to uniformly sample the spectral content of the light beam110′ at the sensor, to evenly distribute the intensity of the light beam110′ at the sensor, and to provide a more accurate measurement of thespectral feature from the sensor. In such implementations, thecoherence-area matching apparatus 315 would still be configured toreduce speckle noise and to improve beam homogenization of the lightbeam 110′ impinging upon the sensor of the spectral detection system310. Even using a single wavefront modification device 516A, thecoherence-area matching apparatus 315 is able to mix the spectralcontent of the light beam 110′ and smooth out the intensity profile ofthe light beam 110′ prior to the light beam 110′ entering the etalonspectrometer. Additionally, the coherence-area matching apparatus 315that uses a single wavefront modification device 516A can modify thelight beam 110′ so that its spatial modes (which are its transverseelectromagnetic modes) are overlapping at a beam homogenization plane(BHP) prior to entering the spectral detection system 310.

In this example, the microlenses 618A, 618B have a hexagonal shape(along the X-Y plane) and they are arranged for a high fill factor,which means that there is very little substrate exposed between eachmicrolens 618A, 618B. The fill factor is measured in terms of thepercentage of area covered by microlenses 618A, 618B to the area ofexposed substrate 619A, 619B between the microlenses 618A, 618B and thefill factor can be at least 90%. The microlenses 618A, 618B can beplano-convex shapes, in which the planar side faces the respectivesubstrate 619A, 619B. The dimensions of the array 616A, 616B aredetermined by the transverse size of the light beam 110′ and the size ofthe transverse spatial modes of the light beam 110′

Moreover, as discussed above, an area A(C) of each microlens 618A, 619Bis matched to a size A(SM) of each spatial mode 417 of the light beam110′. The area A(C) of the microlens 618A, 618B is directly related tothe pitch, P. The spatial modes 417 of the light beam 110′ are formed bythe design and boundary conditions within the resonator or resonators ofthe optical source 105. A spatial mode of the light beam 110′ is aparticular electromagnetic field pattern of radiation measured in aplane perpendicular (that is, transverse) to the propagation directionof the light beam 110′. A spatial mode (which is a transverse mode)manifests itself as the spatial intensity distribution and each spatialmode is associated with a distinct wavelength. Each spatial mode definesa spatially correlated field pattern, and can be considered as acoherence cell. Coherence in this context refers to the spatialcoherence, which describes the correlation (or predictable relationship)between waves at different points in space, either laterally(perpendicular to the direction of the light beam 110′) orlongitudinally (parallel with the direction of the light beam 110′).Thus, spatial coherence describes the ability for two points in space inthe extent of the wave (of the light beam 110′) to interfere, whenaveraged over time. The spatial coherence can be considered to be ameasure of phase relationships in the wavefront transverse to thedirection of propagation of the light beam 110′. The area of a coherencecell is a region of the wavefront within which all points have fixedphase relationships.

In some implementations, the pitch P is a value of about 0.1 to about0.2 mm to closely match a spatial mode length of about 0.15 mm. In oneexample, the array 616A, 616B has a dimension of about 10.8 mm×10.8 mmtaken along the X-Y plane and the array 616A, 616B can be arranged witha grid of about 80-90 microlenses 618A, 618B along each of the X and Ydirections. A pitch P of 0.15 mm for a hexagonal microlens 618Agenerally corresponds to an area A(C) of about 0.017 mm². The estimatedsize of the transverse spatial mode for the light beam 110′ at the planeof the microlens array 616A is about 0.3 mm×0.1 mm. In this example, thesize of the transverse spatial mode corresponds to an area of about0.016 mm². As discussed above, the size of the transverse spatial modeof the light beam 110′ at the output of the beam separator 160 is about0.7 mm×0.1 mm in the orthogonal transverse directions, but this size ofthe transverse spatial mode of the light beam 110′ can be reduced by afactor of about 2.3 along one of the orthogonal transverse direction bybeing interacted with a wavefront modification optic before illuminatingthe microlens array 616A.

Moreover, it is possible for there to be a different number ofmicrolenses 618A, 618B in the grid along the X direction as there arealong the Y direction or that the shape of the microlenses 618A, 619Bhas a varying pitch, P.

Moreover, a spacing or distance D between the arrays 616A and 616B canbe adjustable with an actuator that is physically connected to more oneor more of the arrays 616A and 616B. In the example shown, the distanceD is along the Z direction. The actuator can be connected to the controlsystem 185.

Referring again to FIG. 5, the spatial adjustment system 530 is a lensthat is positioned so that its focal plane overlaps the beamhomogenization plane BHP of the coherence-area matching apparatus 615.In this example, a moving (for example, spinning) diffuser 535 is placedat the beam homogenization plane BHP. The spinning diffuser 535 isdiscussed below. Referring also to FIG. 7, the focal length of eachmicrolens 618A, 618B and the aperture size of each microlens 618A, 618Bdetermine a divergence of the light beam 110′ that is sampled by eachmicrolens 618A, 618B. This divergence, along with a focal length of thelens 630 determine a size or area (taken along the transverse directionto the direction of the light beam 110′) of the light beam 110′ at thebeam homogenization plane BHP if the array 616B is located at the focalplane of the array 616A (and thus, the distance D is equivalent to thefocal length of the array 616A). If the distance D between the array616A and the array 616B is varied, then the size of the light beam 110′in the transverse direction at the beam homogenization plane BHP isvaried. By varying the transverse size of the light beam 110′ at thebeam homogenization plane BHP, it is possible to change (for example,attenuate) a fluence level on the sensor 550 within the spectraldetection system 510 (FIG. 5). The array 616B functions to increase thefield of view or acceptance angle of the coherence-area matchingapparatus 515.

In this example, the curved or convex surfaces of the microlenses 618A,618B can be facing each other (as shown in FIG. 6). In particular, ifthe microlens 618A, 618B are plano-convex lenses, then the microlensarrays 616A, 616B can be oriented such that the convex surfaces of themicrolenses 618A of the array 616A are closest to the convex surfaces ofthe microlenses 618B of the array 616B. Such a design can be useful ifthe focal length of each microlens is relatively short in comparison toa thickness T of the respective substrate 619A, 619B. In one example,the thickness T of the substrate 619A or 619B is about 2-3 mm, whereasthe focal length of a microlens 618A or 618B is about 6 mm. By arrangingthe curved surfaces of the microlenses 618A, 618B facing each other, itis possible to ensure that the focal plane of the array 616A remainsexternal to the substrate 619B of the other array 616B. Such aconfiguration can provide a larger range of fluence attenuation at thesensor 550 of the spectral detection system 510 because the twomicrolens surfaces of the arrays 616A, 616B can be as close as touching.

In an example in which the focal length of each microlens is on theorder of the thickness T, then it is possible to orient the arrays 616A,616B so that the convex surfaces of the microlenses 618A, 618B are notfacing each other. This orientation can be useful in a situation inwhich the focal length of the microlens 616A is much greater than thethickness T of the substrate 619A. Moreover, in a situation in which thefocal length of the microlens 616A is on the order of the thickness T ofthe substrate 619A, then the two microlens arrays 616A, 616B could bemade from a single substrate, as discussed with respect to FIG. 13.

As shown in FIG. 7, the microlenses 618A, 618B of the pair of arrays616A, 616B are spaced (by a value of D) and sized along the X-Y plane sothat each spatial mode of the pulsed light beam 110′ that passes throughthe coherence-area matching apparatus 615 is projected to the same areaat the focal plane of the lens 530, the focal plane overlapping the beamhomogenization plane BHP. The shape of the light beam 110′, which ishomogenized, at the beam homogenization plane BHP is the same as theshape of the microlenses 618A, 618B and thus would be a hexagonal shape.The sides of the hexagonal shape at the beam homogenization plane BHPhave a length of about 2-4 mm if the array 616B is located at the focalplane of the array 616A (in which case, D would equal the focal lengthof the array 616A).

Referring again to FIG. 5, the spinning diffuser 535 is placed at thebeam homogenization plane BHP, which is the plane at which the lightbeam 110′ has been homogenized. The spinning diffuser 535 is a diffuserthat is rotated about the direction of the path of the light beam 110′.The diffuser 535 diffuses the light beam 110′ to a cone to fill anaperture 561 of the spectral detection system 510. The spinning diffuser535 also reduces any spikes in the intensity within the light beam 110′that can result from interference of the copies of the spatial modessampled within the coherence-area matching apparatus 515. Moreover, theaperture 561 is placed at a focal plane FP (562) of an input lens 562within the spectral detection system 510. By locating the aperture 561of the spectral detection system 510 at the focal plane FP (562) of theinput lens 562, each point from the focal plane FP (562) acts as a pointsource and accordingly, the input lens 562 acts to collimate the lightbeam 110′ before entering an etalon 563. An output lens 564 ispositioned at the exit of the etalon 563 so that its focal plane FP(564) overlaps the active area of the sensor 550.

In some implementations, the etalon 563 includes a pair of partiallyreflective glass or optical flats 563A, 563B, which can be spaced ashort distance (for example, millimeters to centimeters) apart, with thereflective surfaces facing each other. In other implementations, theetalon 563 includes a single plate with two parallel reflectingsurfaces. The flats 563A, 563B can be made in a wedge shape to preventthe rear surfaces from producing interference fringes; the rear surfacesoften also have an anti-reflective coating. As the light beam 110′passes through the paired flats 563A, 563B, it is multiply reflected,and produces a plurality of transmitted rays, which are collected by theoutput lens 564 and brought to the active region of the sensor 550.Interference effects, dependent on the direction of the transmittedrays, produce constructive and destructive interference of the differentspectral components of the light beam 110′, such that only selectspectral components are transmitted along the direction of a given ray.In this manner, the spectral content of the light beam 110′ is mappedinto the spatial direction of the transmitted rays. The spectraldetection system 510 also includes an optical delay 580, as needed, toensure that the sensor 550 is at the focal plane of the output lens 564.

Referring also to FIG. 8, more details of the spectral detection system510 are provided. The etalon 563 interacts with the light beam 110′ andoutputs a plurality of spatial components 574 that correspond to thespectral components of the light beam 110′. The spectral components ofthe light beam 110′ are in the optical spectrum 572 of the light beam110′; therefore, they correspond to how the optical energy or power ofthe light beam 110′ is distributed over the different wavelengths. Thespatial components 574 correspond to these intensities mapped into atwo-dimensional space. Thus, the etalon 563 transforms the spectralinformation (such as the wavelength) of the light beam 110′ into spatialinformation that can be sensed or detected by the sensor 550. Thetransformation maps the spectral information (such as the wavelength) todifferent positions in space so that the spectral information that canbe observed by the sensor 550.

The etalon 563 produces as the spatial components 574 an interferencepattern that takes the appearance of a set of concentric rings. Theinterference pattern takes the appearance of a more uniform intensitydistribution if the intensity distribution of the light beam 110′ on theaperture 561 is more uniform. In particular, the sharpness of the ringsdepends on the reflectivity of the flats 563A, 563B of the etalon 563.Thus, if the reflectivity of the flats 563A, 563B is high (such that theetalon has a high quality (Q) factor), when the beam 110′ is amonochromatic light beam, the etalon 563 produces a set of narrow brightrings against a dark background. To put it another way, even if twospectral components 574 of the light beam 110′ are equally representedin the optical spectrum, the peak intensity of the respectiveinterference pattern rings will not be equal unless the input light beamto the etalon 563 uniformly illuminates both of the corresponding raydirections. The transmission of the etalon 563 as a function ofwavelength is shown in the resulting fringe pattern 571, which producesthe optical spectrum 572 that is directed to the control system 185.

While the complete interference pattern is shown, it is not needed toperform the calculations or estimates; it is alternatively possible togenerate only fringes within a region that is slightly larger than anactive area of the sensor 550.

The sensor 550 receives and senses the output spatial components 574.The sensor 550 can be defined by a linear axis that indicates generallythe active area of its sensing region. The linear axis of the sensingregion can be perpendicular to the direction of propagation of thespatial components 574.

The sensor 550 can be a detector that receives and senses the outputspatial components 574. For example, one type of suitable detector thatcan be used to measure along one dimension is a linear photodiode array.The linear photodiode array is consists of multiple elements of the samesize, formed in a linear arrangement at an equal spacing in one package.The photodiode array is sensitive to the wavelengths contained in thelight beam 110′; thus, if the light beam 110′ has an optical spectrumcontaining only wavelengths in the deep ultraviolet range, then thephotodiode array is sensitive to light having a wavelength in the deepultraviolet range. As another example, the sensor 550 can be a twodimensional sensor such as a two-dimensional charged coupled device(CCD) or a two-dimensional complementary metal oxide semiconductor(CMOS) sensor. Again, if the light beam 110′ has an optical spectrumcontaining only wavelengths in the deep ultraviolet range, then thetwo-dimensional sensor 550 is sensitive to light having a wavelength inthe deep ultraviolet range. The sensor 550 should be able to read outdata at a fast enough rate, for example, at about 6 kHz.

The control system 185 is connected to the output of the sensor 550 aswell as the optical source 105 and the spectral feature selection system130 that is optically coupled to the light beam 110. The control system185 measures a property of the spatial components 574, and analyzesthese measured properties to calculate an estimate of the spectralfeature of the light beam 110. The control system 185 can perform themeasurement, analysis, and calculation for each pulse of the light beam110 or for a set of pulses of the light beam 110.

The property P that is measured can be a scalar quantity (which is fullydescribed by a magnitude or numerical value) alone or a vector quantity(which is fully described by both a magnitude and a direction). Anexample of a scalar property P is a metric such as the width of theoptical spectrum 572. In this example, it is possible that the entireshape of the optical spectrum 572 is not known but the metric is knownand this is used to estimate the shape of the optical spectrum 572. Anexample of a vector property P is the entire waveform that describes theoptical spectrum 572. In this example, one can calculate any metric fromthe entire spectrum and the by having the entire spectrum, one can makea more accurate calculation. The sensed spatial components can bemeasured for a range of one or more pulses of the pulsed light beam110′.

The control system 185 can measure as the property P the width W of theoptical spectrum 572. The width W of the optical spectrum 572 canprovide an estimate of the bandwidth (the spectral feature) of the lightbeam 110′. In some implementations, the width W of the optical spectrum572 is determined using a metric such as the FWXM (full width of thespectrum 572 at a fraction X of the maximum peak intensity). In otherimplementations, the width W of the optical spectrum 572 is determinedusing a metric such as EY (the width of the spectrum that contains afraction Y of the integrated spectral intensity). Other metrics aresuitable for measuring the property of the optical spectrum 572.

Referring to FIG. 9, in some implementations, the optical source 105 isan exemplary optical source 905. The optical source 905 is a pulsedlaser source that produces a pulsed laser beam as the light beam 110.The optical source 905 is a two-stage laser system that includes amaster oscillator (MO) 900 that provides a seed light beam 910A to apower amplifier (PA) 910. The master oscillator 900 typically includes again medium in which amplification occurs and an optical feedbackmechanism such as an optical resonator. The power amplifier 910typically includes a gain medium in which amplification occurs whenseeded with the seed laser beam from the master oscillator 900. If thepower amplifier 910 is designed as a regenerative ring resonator then itis described as a power ring amplifier (PRA) and in this case, enoughoptical feedback can be provided from the ring design. The spectralfeature selection apparatus 130 receives the light beam 110A from themaster oscillator 900 to enable fine tuning of spectral parameters suchas the center wavelength and the bandwidth of the light beam 110A atrelatively low output pulse energies. The power amplifier 910 receivesthe seed light beam 910A from the master oscillator 900 and amplifiesthis output to attain the necessary power for output to use inphotolithography.

The master oscillator 900 includes a discharge chamber having twoelongated electrodes, a laser gas that serves as the gain medium, and afan circulating the gas between the electrodes. A laser resonator isformed between the spectral feature selection apparatus 130 on one sideof the discharge chamber, and an output coupler 915 on a second side ofthe discharge chamber to output the seed light beam 910A to the poweramplifier 910.

The optical source 905 can also include another spectral measurementmodule 920 that receives an output from the output coupler 915, and oneor more beam modification optical systems 925 that modify the sizeand/or shape of the beam as needed. The spectral measurement module 920is an example of another type of metrology system (such as the metrologysystem 170) that can be used to measure the wavelength (for example, thecenter wavelength) of the seed light beam 910A.

The power amplifier 910 includes a power amplifier discharge chamber,and if it is a regenerative ring amplifier, the power amplifier alsoincludes a beam reflector or beam turning device 930 that reflects thelight beam back into the discharge chamber to form a circulating path.The power amplifier discharge chamber includes a pair of elongatedelectrodes, a laser gas that serves as the gain medium, and a fan forcirculating the gas between the electrodes. The seed light beam 910A isamplified by repeatedly passing through the power amplifier 910. Thebeam modification optical system 925 provides a way (for example, apartially-reflecting mirror) to in-couple the seed light beam 910A andto out-couple a portion of the amplified radiation from the poweramplifier to form the output light beam 110.

The laser gas used in the discharge chambers of the master oscillator900 and the power amplifier 910 can be any suitable gas for producing alaser beam around the required wavelengths and bandwidth. For example,the laser gas can be argon fluoride (ArF), which emits light at awavelength of about 193 nm, or krypton fluoride (KrF), which emits lightat a wavelength of about 248 nm.

The spectral measurement module 920 monitors the wavelength of theoutput (the seed light beam 910A) of the master oscillator 900. Thespectral measurement module 920 can be placed at other locations withinthe optical source 905, or it can be placed at the output of the opticalsource 905.

The repetition rate of the pulses produced by the power amplifier 910 isdetermined by the repetition rate at which the master oscillator 900 iscontrolled by the control system 185, under the instructions from thecontroller 140 in the scanner 115. The repetition rate of the pulsesoutput from the power amplifier 910 is the repetition rate seen by thescanner 115.

As discussed above, it is possible to control the bandwidth bothcoarsely and finely using only optical elements. On the other hand, itis possible to control the bandwidth in a fine and narrow range, andrapidly, by controlling a differential timing between the activation ofthe electrodes within the MO 900 and the PRA 910 while controlling thebandwidth in a coarse and wide range by adjusting the angle of a prismwithin the spectral feature selection system 130.

Referring to FIG. 10, in some implementations, the spectral featureselection apparatus 130 includes a set of optical features or components1000, 1005, 1010, 1015, 1020 arranged to optically interact with thepulsed light beam 110A and a control module 1050 that includeselectronics in the form of any combination of firmware and software. Theoptical components 1000, 1005, 1010, 1015, 1020 can be configured toprovide a coarse spectral feature adjustment system; and, if theadjustment of such components is rapid enough, it can be configured toprovide a fine spectral feature adjustment system. Although not shown inFIG. 10, it is possible for the spectral feature selection apparatus 130to include other optical features or other non-optical features forproviding fine spectral feature control.

The control module 1050 is connected to one or more actuation systems1000A, 1005A, 1010A, 1015A, 1020A physically coupled to respectiveoptical components 1000, 1005, 1010, 1015, 1020. The optical componentsof the apparatus 130 include a dispersive optical element 1000, whichcan be a grating, and a beam expander 1001 made of a set of refractiveoptical elements 1005, 1010, 1015, 1020, which can be prisms. Thegrating 1000 can be a reflective grating that is designed to disperseand reflect the light beam 110A; accordingly, the grating 1000 is madeof a material that is suitable for interacting with a pulsed light beam110A having a wavelength in the DUV range. Each of the prisms 1005,1010, 1015, 1020 is a transmissive prism that acts to disperse andredirect the light beam 110A as it passes through the body of the prism.Each of the prisms can be made of a material (such as, for example,calcium fluoride) that permits the transmission of the wavelength of thelight beam 110A. Although four refractive optical elements 1005, 1010,1015, 1020 are shown, it is possible for fewer than four or more thanfour to be used in the beam expander 1001.

The pulsed light beam 110A enters the apparatus 130 through an aperture1055, and then travels through the prism 1020, the prism 1010, and theprism 1005, in that order, prior to impinging upon a diffractive surface1002 of the grating 1000. With each passing of the beam 110A through aconsecutive prism 1020, 1015, 1010, 1005, the light beam 110A isoptically magnified and redirected (refracted at an angle) toward thenext optical component. The light beam 110A is diffracted and reflectedfrom the grating 1000 back through the prism 1005, the prism 1010, theprism 1015, and the prism 1020, in that order, prior to passing throughthe aperture 1055 as the light beam 110A exits the apparatus 130. Witheach passing through the consecutive prisms 1005, 1010, 1015, 1020 fromthe grating 1000, the light beam 110A is optically compressed as ittravels toward the aperture 1055.

The rotation of a prism (which can be any one of prisms 1005, 1010,1015, or 1020) of the beam expander 1001 changes an angle of incidenceat which the light beam 110A impinges upon the entrance surface of thatrotated prism. Moreover, two local optical qualities, namely, an opticalmagnification and a beam refraction angle, of the light beam 110Athrough that rotated prism are functions of the angle of incidence ofthe light beam 110A impinging upon the entrance surface of that rotatedprism. The optical magnification of the light beam 110A through theprism is the ratio of a transverse wide of the light beam 110A exitingthat prism to a transverse width of the light beam 110A entering thatprism.

A change in the local optical magnification of the light beam 110A atone or more of the prisms within the beam expander 1001 causes anoverall change in the optical magnification OM 1065 of the light beam110A through the beam expander 1001. The optical magnification OM 1065of the light beam 110A through the beam expander 1001 is the ratio ofthe transverse width Wo of the light beam 110A exiting the beam expander1001 to a transverse width Wi of the light beam 110A entering the beamexpander 1001. Additionally, a change in the local beam refraction anglethrough one or more of the prisms within the beam expander 1001 causesan overall change in an angle of incidence of 1062 of the light beam110A at the surface 1002 of the grating 1000.

The wavelength of the light beam 110A can be adjusted by changing theangle of incidence 1062 at which the light beam 110A impinges upon thediffractive surface 1002 of the grating 1000. The bandwidth of the lightbeam 110A can be adjusted by changing the optical magnification 1065 ofthe light beam 110.

The apparatus 130 is designed to adjust the wavelength of the light beam110A that is produced within the resonator or resonators of the opticalsource 105 by adjusting an angle 1062 of incidence of at which the lightbeam 110A impinges upon the diffractive surface 1002 of the grating1000. Specifically, this can be done by rotating one or more of theprisms 1005, 1010, 1015, 1020 and the grating 1000 to thereby adjust theangle of incidence 1062 of the light beam 110A.

Moreover, the bandwidth of the light beam 110A that is produced by theoptical source 105 is adjusted by adjusting the optical magnification OM1065 of the light beam 110A. Thus, the bandwidth of the light beam 110Acan be adjusted by rotating one or more of the prisms 1005, 1010, 1015,1020, which causes the optical magnification 1065 of the light beam 110Ato change. Because the rotation of a particular prism causes a change inboth the local beam refraction angle and the local optical magnificationat that prism, the control of wavelength and bandwidth are coupled inthis design.

Additionally, the bandwidth of the light beam 110A is relativelysensitive to the rotation of the prism 1020 and relatively insensitiveto rotation of the prism 1005. This is because any change in the localoptical magnification of the light beam 110A due to the rotation of theprism 1020 is multiplied by the product of the change in the opticalmagnification in the other prisms 1015, 1010, and 1005 because thoseprisms are between the rotated prism 1020 and the grating 1000, and thelight beam 110A must travel through these other prisms 1015, 1010, 1005after passing through the prism 1020. On the other hand, the wavelengthof the light beam 110A is relatively sensitive to the rotation of theprism 1005 and relatively insensitive to the rotation of the prism 1020.

For example, in order to change the bandwidth without changing thewavelength, the optical magnification 1065 should be changed withoutchanging the angle of incidence 1062, and this can be achieved byrotating the prism 1020 by a large amount and rotating the prism 1005 bya small amount.

The control module 1050 is connected to one or more actuation systems1000A, 1005A, 1010A, 1015A, 1020A that are physically coupled torespective optical components 1000, 1005, 1010, 1015, 1020. Although anactuation system is shown for each of the optical components it ispossible that some of the optical components in the apparatus 130 areeither kept stationary or are not physically coupled to an actuationsystem. For example, in some implementations, the grating 1000 can bekept stationary and the prism 1015 can be kept stationary and notphysically coupled to an actuation system.

Each of the actuation systems 1000A, 1005A, 1010A, 1015A, 1020A includesone or more actuators that are connected to its respective opticalcomponents. The adjustment of the optical components causes theadjustment in the particular spectral features (the wavelength and/orbandwidth) of the light beam 110A. The control module 1050 receives acontrol signal from the control system 185, the control signal includingspecific commands to operate or control one or more of the actuationsystems. The actuation systems can be selected and designed to workcooperatively.

Each of the actuators of the actuation systems 1000A, 1005A, 1010A,1015A, 1020A is a mechanical device for moving or controlling therespective optical component. The actuators receive energy from themodule 1050, and convert that energy into some kind of motion impartedto the respective optical component. For example, the actuation systemscan be any one of force devices and rotation stages for rotating one ormore of prisms of a beam expander. The actuation systems can include,for example, motors such as stepper motors, valves, pressure-controlleddevices, piezoelectric devices, linear motors, hydraulic actuators,voice coils, etc.

The grating 1000 can be a high blaze angle Echelle grating, and thelight beam 110A incident on the grating 1000 at any angle of incidence1062 that satisfies a grating equation will be reflected (diffracted).The grating equation provides the relationship between the spectralorder of the grating 1000, the diffracted wavelength (the wavelength ofthe diffracted beam), the angle of incidence 1062 of the light beam 110Aonto the grating 1000, the angle of exit of the light beam 110Adiffracted off the grating 1000, the vertical divergence of the lightbeam 110A incident onto the grating 1000, and the groove spacing of thediffractive surface of the grating 1000. Moreover, if the grating 1000is used such that the angle of incidence 1062 of the light beam 110Aonto the grating 1000 is equal to the angle of exit of the light beam110A from the grating 1000, then the grating 1000 and the beam expander(the prisms 1005, 1010, 1015, 1020) are arranged in a Littrowconfiguration and the wavelength of the light beam 110A reflected fromthe grating 1000 is the Littrow wavelength. It can be assumed that thevertical divergence of the light beam 110A incident onto the grating1000 is near zero. To reflect the nominal wavelength, the grating 1000is aligned, with respect to the light beam 110A incident onto thegrating 1000, so that the nominal wavelength is reflected back throughthe beam expander (the prisms 1005, 1010, 1015, 1020) to be amplified inthe optical source 105. The Littrow wavelength can then be tuned overthe entire gain bandwidth of the resonators within optical source 105 byvarying the angle of incidence 1062 of the light beam 110A onto thegrating 1000.

Each of the prisms 1005, 1010, 1015, 1020 is wide enough along thetransverse direction of the light beam 110A so that the light beam 110Ais contained within the surface at which it passes. Each prism opticallymagnifies the light beam 110A on the path toward the grating 1000 fromthe aperture 1055, and therefore each prism is successively larger insize from the prism 1020 to the prism 1005. Thus, the prism 1005 islarger than the prism 1010, which is larger than the prism 1015, and theprism 1020 is the smallest prism.

As discussed above, the bandwidth of the light beam 110A is relativelysensitive to the rotation of the prism 1020 and relatively insensitiveto rotation of the prism 1005. This is because any change in the localoptical magnification of the light beam 110A due to the rotation of theprism 1020 is multiplied by the product of the change in the opticalmagnification in the other prisms 1015, 1010, and 1005 because thoseprisms are between the rotated prism 1020 and the grating 1000, and thelight beam 110A must travel through these other prisms 1015, 1010, 1005after passing through the prism 1020. On the other hand, the wavelengthof the light beam 110A is relatively sensitive to the rotation of theprism 1005 and relatively insensitive to the rotation of the prism 1020.Thus, the wavelength can be coarsely changed by rotating the prism 1005,and the prism 1020 can be rotated (in a coarse manner). The angle ofincidence 1062 of the light beam 110A is changed due to the rotation ofthe prism 1005 and the rotation of the prism 1020 offset the change inmagnification caused by the rotation of the prism 1005. The prism 1020can be used for coarse, large range, and slow bandwidth control. Bycontrast, the bandwidth can be controlled in a fine and narrow range andeven more rapidly by controlling the prism 1010.

Referring to FIG. 11, details about the control system 185 are providedthat relate to the aspects of the system and method described herein.The control system 185 can include other features not shown in FIG. 11.In general, the control system 185 includes one or more of digitalelectronic circuitry, computer hardware, firmware, and software.

The control system 185 includes memory 1100, which can be read-onlymemory and/or random access memory. Storage devices suitable fortangibly embodying computer program instructions and data include allforms of non-volatile memory, including, by way of example,semiconductor memory devices, such as EPROM, EEPROM, and flash memorydevices;

-   -   magnetic disks such as internal hard disks and removable disks;        magneto-optical disks; and CD-ROM disks. The control system 185        can also include one or more input devices 1105 (such as a        keyboard, touch screen, microphone, mouse, hand-held input        device, etc.) and one or more output devices 1110 (such as a        speaker or a monitor).

The control system 185 includes one or more programmable processors1115, and one or more computer program products 1120 tangibly embodiedin a machine-readable storage device for execution by a programmableprocessor (such as the processors 1115). The one or more programmableprocessors 1115 can each execute a program of instructions to performdesired functions by operating on input data and generating appropriateoutput. Generally, the processor 1115 receives instructions and datafrom memory 1100. Any of the foregoing may be supplemented by, orincorporated in, specially designed ASICs (application-specificintegrated circuits).

The control system 185 includes, among other components, a spectralfeature analysis module 1125, a lithography analysis module 1130, adecision module 1135, a light source actuation module 1150, alithography actuation module 1155, and a beam preparation actuationmodule 1160. Each of these modules can be a set of computer programproducts executed by one or more processors such as the processors 1115.Moreover, any of the modules 1125, 1130, 1135, 1150, 1155, 1160 canaccess data stored within the memory 1100.

The spectral feature analysis module 1125 receives the output from themetrology system 170 and the spectral measurement module 920. Thelithography analysis module 1130 receives information from thelithography controller 140 of the scanner 115. The decision module 1135receives the outputs from the analyses modules (such as the modules 1125and 1130) and determines which actuation module or modules need to beactivated based on the outputs from the analyses modules. The lightsource actuation module 1150 is connected to one or more of the opticalsource 105 and the spectral feature selection apparatus 130. Thelithography actuation module 1155 is connected to the scanner 115, andspecifically to the lithography controller 140. The beam preparationactuation module 1160 is connected to one or more components of the beampreparation system 112.

While only a few modules are shown in FIG. 11, it is possible for thecontrol system 185 to include other modules. Additionally, although thecontrol system 185 is represented as a box in which all of thecomponents appear to be co-located, it is possible for the controlsystem 185 to be made up of components that are physically remote fromeach other. For example, the light source actuation module 1150 can bephysically co-located with the optical source 105 or the spectralfeature selection apparatus 130.

In general, the control system 185 receives at least some informationabout the light beam 110 from the metrology system 170 and/or thespectral measurement module 920, and the spectral feature analysismodule 1125 performs an analysis on the information to determine how toadjust one or more spectral features (for example, the bandwidth) of thelight beam 110 supplied to the scanner 115. Based on this determination,the control system 185 sends signals to the spectral feature selectionapparatus 130 and/or the optical source 105 to control operation of theoptical source 105 via the control module 1050. In general, the spectralfeature analysis module 1125 performs the analysis needed to estimateone or more spectral features (for example, the wavelength and/or thebandwidth) of the light beam 110. The output of the spectral featureanalysis module 1125 is an estimated value of the spectral feature thatis sent to the decision module 1135.

The spectral feature analysis module 1125 includes a comparison blockconnected to receive the estimated spectral feature and also connectedto receive a spectral feature target value. In general, the comparisonblock outputs a spectral feature error value that represents adifference between the spectral feature target value and the estimatedvalue. The decision module 1135 receives the spectral feature errorvalue and determines how best to effect a correction to the system 100in order to adjust the spectral feature. Thus, the decision module 1135sends a signal to the light source actuation module 1150, whichdetermines how to adjust the spectral feature selection apparatus 130(or the optical source 105) based on the spectral feature error value.The output of the light source actuation module 1150 includes a set ofactuator commands that are sent to the spectral feature selectionapparatus 130. For example, the light source actuation module 1150 sendsthe commands to the control module 1050, which is connected to theactuation systems within the apparatus 1030.

Additionally, the lithography analysis module 1130 can receiveinstructions from the lithography controller 140 of the scanner 115 forexample, to change one or more spectral features of the pulsed lightbeam 110 or to change a pulse repetition rate of the light beam 110. Thelithography analysis module 1130 performs an analysis on theseinstructions to determine how to adjust the spectral features and sendsthe results of the analysis to the decision module 1135. The controlsystem 185 causes the optical source 105 to operate at a givenrepetition rate. More specifically, the scanner 115 sends a triggersignal to the optical source 105 (by way of the control system (throughthe lithography analysis module 1130) for every pulse (that is, on apulse-to-pulse basis) and the time interval between those triggersignals can be arbitrary, but when the scanner 115 sends trigger signalsat regular intervals then the rate of those signals is a repetitionrate. The repetition rate can be a rate requested by the scanner 115.

Referring to FIG. 12, a procedure 1200 is performed by thephotolithography system 100 to estimate a spectral feature of the lightbeam 110′. The light beam 110′ is homogenized (1205). As discussedabove, the light beam 110′ is homogenized by passing the light beam 110′through a pair of arrays of wavefront modification cells, with each cellhaving a surface area that matches a size of the spatial modes of thelight beam 110′. In this way, each transverse spatial mode of the lightbeam 110′ is projected to the same transverse area at a beamhomogenization plane.

The homogenized light beam is interacted with an optical frequencyseparation apparatus (such as the etalon 563 within the metrology system170 or an optical component within the spectral detection system 1410 ofthe spectral analysis module 920) that outputs spatial components thatcorrespond to the spectral components of the light beam (1210). Forexample, the homogenized light beam is directed through the etalon 563,which transforms the spectral information (such as the wavelength) ofthe light beam 110′ into spatial information. The outputted spatialcomponents are sensed (1215), for example, by the sensor 550. Thecontrol system 185 receives the output of the sensor 550, and measures aproperty of the sensed spatial components (1220). The control system 185analyzes the measured properties to estimate a spectral feature of thepulsed light beam (1225), determines whether the estimated spectralfeature of the pulsed light beam is within an acceptable range ofspectral features (1230).

Moreover, if the control system 185 determines that the estimatedspectral feature of the pulsed light beam is outside the acceptablerange (1230), then the control system 185 sends an adjustment signal tothe spectral feature selection system 130 to modify the spectral featureof the pulsed light beam 110.

Other implementations are within the scope of the following claims. Forexample, in other implementations, the metrology system 170 includesother features not shown or discussed for measuring other aspects of thelight beam 110. In other implementations, the microlenses 618A, 618B andthe respective substrates 619A, 619B are made of fused silica, aluminumfluoride, encapsulated magnesium fluoride, gadolinium fluoride, orsodium aluminum fluoride.

Referring to FIG. 13, in another implementation, the coherence-areamatching apparatus 515 includes, as the wavefront modification devices516A and 516B, two microlens arrays 1316A, 1316B, and each array 1316A,1316B is applied to a single support substrate 1319. In this way, thecurved or convex surfaces of the microlenses of each array can be facingaway from each other (as shown in FIG. 13). In particular, if themicrolens are plano-convex lenses, then the microlens arrays 1316A,1316B can be oriented such that the plane surfaces of the microlenses ofthe array 1316A are closest to the plane surfaces of the microlenses ofthe array 1316B.

With reference to FIG. 14, in other implementations, it is possible touse the coherence-area matching apparatus 315 or the entire beamhomogenizer 305 to reduce the spatial coherence of the light beam 110 atother regions of the photolithography system 100. For example, the beamhomogenizer 305 or just the coherence-area matching apparatus 315 couldbe placed in the path of a portion 910A′ of the seed light beam 910Aoutput from the master oscillator 900. In this implementation, the beamhomogenizer 305 is configured to reduce the spatial coherence of theseed light beam portion 910A′ directed to a spectral detection system1410 within the spectral analysis module 920. The seed light beamportion 910A′ is split off from the seed light beam portion 910A by abeam separator 1460, which directs the seed light beam portion 910A′toward a diagnostic apparatus 1465 that includes the spectral detectionsystem 1410 and the beam homogenizer 305. In this example, the spectraldetection system 1410 can be used to measure or detect a spectralfeature such as the wavelength of the seed light beam portion 910A′ forfurther diagnosis by the control system 185.

What is claimed is:
 1. A metrology system for measuring a spectralfeature of a pulsed light beam, the system comprising: a beamhomogenizer in the path of the pulsed light beam, the beam homogenizerhaving an array of wavefront modification cells, with each cell having asurface area that matches a size of at least one of the spatial modes ofthe light beam; an etalon in the path of the pulsed light beam exitingthe beam homogenizer, wherein the etalon is configured to interact withthe pulsed light beam and to output a plurality of spatial componentsthat correspond to the spectral components of the pulsed light beam; atleast one sensor that receives and senses the output spatial components.2. The system of claim 1, further comprising a control system connectedto an output of the at least one sensor and configured to: measure aproperty of the output spatial components from the etalon for one ormore pulses; analyze the measured property to calculate an estimate ofthe spectral feature of the pulsed light beam; and determine whether theestimated spectral feature of the pulsed light beam is within anacceptable range of values of spectral features.
 3. The system of claim2, wherein the spectral feature is a bandwidth of the pulsed light beam.4. The system of claim 2, further comprising a spectral featureselection system optically connected to the pulsed light beam, wherein:the control system is connected to the spectral feature selectionsystem; and if the control system determines that the estimated spectralfeature of the pulsed light beam is outside the acceptable range, thenthe control system is configured to send an adjustment signal to thespectral feature selection system to modify the spectral feature of thepulsed light beam.
 5. The system of claim 1, wherein a cell surface areamatches a mode size of the light beam if the cell surface area isbetween 0.5 and 1.5 times the area of the spatial mode.
 6. The system ofclaim 1, wherein a cell surface area matches a mode size of the lightbeam if the cell surface area is between 0.9 and 1.1 times the area ofthe spatial mode.
 7. The system of claim 1, further comprising anoptical diffuser in the path of the light beam, wherein the beamhomogenizer receives the light beam that is output from the opticaldiffuser.
 8. The system of claim 7, wherein the optical diffuserincludes a microlens array.
 9. The system of claim 1, further comprisinga beam separation device in the path between a source that produces thelight beam and a photolithography exposure apparatus, wherein the beamseparation device: directs a first percentage of the light beam towardthe beam homogenizer, and directs a second percentage of the light beamalong the path toward the photolithography exposure apparatus.
 10. Thesystem of claim 9, further comprising an optical temporal pulsestretcher between the beam separation device and the beam homogenizer.11. The system of claim 10, wherein the optical temporal pulse stretcheris a passive optical element.
 12. The system of claim 1, wherein thebeam homogenizer comprises: at least two arrays, each array having aplurality of wavefront modification cells; and a lens that receives thelight beam output of the at least two arrays.
 13. The system of claim12, wherein the homogenized beam plane is at the focal plane of thelens.
 14. The system of claim 13, further comprising a spinning diffuserat the homogenized beam plane.
 15. The system of claim 13, wherein thelens has a focal length that is large enough so that the spacing betweendiffraction spikes of the output light beam from the at least two arraysis greater than an area of the at least one sensor that receives theoutput light beam from the beam homogenizer.
 16. The system of claim 12,further comprising an actuator connected to one or more of the at leasttwo arrays, and configured to adjust a distance between the at least twoarrays.
 17. The system of claim 1, wherein the beam homogenizer cellarray is made of calcium fluoride, fused silica, aluminum fluoride,encapsulated magnesium fluoride, gadolinium fluoride, or sodium aluminumfluoride.
 18. The system of claim 1, wherein the beam homogenizer cellarray comprises an array of lenslets.
 19. The system of claim 1, whereinthe beam homogenizer cell array is a transmissive cell array.
 20. Thesystem of claim 1, wherein the light beam has a plurality ofwavelengths, at least some being in the deep ultraviolet range.
 21. Thesystem of claim 1, wherein the size of the spatial mode of the lightbeam corresponds to a transverse area across the light beam in which allpoints within the transverse area have a fixed phase relationship. 22.The system of claim 1, wherein the beam homogenizer is in the path of apulsed light beam output from a power amplifier of an optical source.23. The system of claim 1, wherein the beam homogenizer is in the pathof a pulsed seed light beam output from a master oscillator of anoptical source.